Update: OverDrive has made the decision to restore the download option for MP3 audiobooks on Mac via your library’s OverDrive website for users who have macOS 10.6 (Snow Leopard) to macOS 10.14 (Mojave). The OverDrive app for Mac will not be restored so it will not be available for download and installation. OverDrive hopes to restore MP3 audio book download by October 2, 2019. Again: these changes do not affect Apple devices that use iOS (iPhone/iPad/iPod Touch). Note: If you upgrade your computer to macOS 10.15 (Catalina), you will lose the MP3 download option and OverDrive for Mac compatibility, but will still be able to listen to MP3 audio books via OverDrive Listen in your internet browser.
Apple recently dropped support for the OverDrive app on Macs (iMac/MacBook); please see this article for further information. OverDrive content can still be enjoyed on a Mac device via the "Read in browser" feature (for ebooks) and "Listen in browser" feature (for audio books).
NOTE: This change does not affect Apple devices that use iOS (iPhone/iPad/iPod Touch).